This invention is in the field of sewing machines, more particularly, it is concerned with a conversion device permitting the conversion of a lock stitch sewing machine to the production of chain stitches.
There are in the prior art, many examples of chain stitch devices for lock stitch sewing machines. There are, for example, the devices disclosed by U.S. Pat. Nos. 3,173,390 and 3,173,391 issued on Mar. 16, 1965 to, respectively, Bartosz and Doerner. These two patents disclose the use of leaf springs to retain the prior thread loop until the sewing needle has stepped therethrough on its way to having a new thread loop seized by the loop taker.
There is also U.S. Pat. No. 3,194,197, issued on July 13, 1965 to Ketterer et al, which patent discloses a lever pivotally connected to the throat plate and having a loop retaining finger with an opening therethrough which may engage with a finger extending from a plate supported in the bobbin case to retain a loop on the loop retaining finger when the lever is urged by a thread pull off to maintain intimate contact between the thread engaging finger and the plate in the bobbin case.
U.S. Pat. No. 3,253,560, issued on May 31, 1966 to Ketterer et al and U.S. Pat. No. 3,720,178 issued on Mar. 13, 1973 to Ivanko disclose inserts for a bobbin case which are rockable or have a portion thereof rockable in order to, at first, retain a thread loop and subsequently release the loop of thread after the sewing needle has passed therethrough.
It frequently occurs that an advance in the art prevents the use of heretofore known devices such as the aforementioned chain stitch conversion devices. Thus, the development of a new anti-rotation device for the bobbin case, which device is located adjacent and in front of the sewing needle, required the development of a new chain stitch conversion device compatible for use therewith.